a) Field of the Invention
This invention relates to a metallic honeycomb body which is generally used in a form interposed as exhaust gas cleaning means in an exhaust pipe system of an automotive vehicle.
More specifically, this invention is concerned with an X-wrapped metallic honeycomb body for carrying, especially an exhaust gas cleaning catalyst thereon, said honeycomb body having an increased carrying surface area in a central part thereof and improved durability.
b) Description of the Related Art
In automotive exhaust gas cleaning means of the above-described type, monolithic ceramic carrier bodies have been used to date. However, monolithic metallic carrier bodies are now attracting interests in place of such conventional carrier bodies.
In general, a metal-made, monolithic carrier body for carrying an exhaust gas cleaning catalyst is constructed from a honeycomb-shaped, multilayered composite body (hereinafter called a "metallic honeycomb body") and a metallic cylindrical casing having opening at opposite ends thereof so that the metallic honeycomb body can be inserted into the casing and then fixed on the casing. The metallic honeycomb body has been formed by stacking planar bands and corrugated bands, each of which is made from a heat-resisting steel sheet, in a mutually contiguous relation and rolling them together into a spiral form and defines a number of network-patterned, axial gas flow passages (hereinafter called "cells") for permitting passage of exhaust gas therethrough.
The metallic honeycomb body and the metallic casing are firmly fixed together by brazing or welding them so that the metallic honeycomb body can withstand thermal expansions and thermal stresses--which occur in a high-temperature atmosphere due to the high temperature of the exhaust gas itself and an exothermic reaction of the exhaust gas by the exhaust gas cleaning catalyst--and also extreme vibrations during running of the automotive vehicle.
Needless to say, the planar bands and corrugated bands which make up the metallic honeycomb body are fixed together at least some of areas of contact therebetween by brazing, welding or mechanical fastening to form the metallic honeycomb body into a vibration proof structure.
The metallic honeycomb body of the rolled type, a principal element of the above-described metallic carrier body, is superior in various aspects to the ceramic-made monolithic honeycomb body. There is however room for a further improvement with respect to large thermal stresses which occur under the severe thermal conditions described above.
In particular, an unduly large thermal stress concentrates on an outer peripheral portion of the metallic honeycomb body, said portion being in contact with an inner peripheral wall of the metallic casing, or a portion adjacent to the outer peripheral portion. The planar bands and corrugated bands of the metallic honeycomb body are therefore buckled, cracked or otherwise damaged so that the durability of the metallic carrier body is reduced.
As replacements for metallic honeycomb bodies of the above-described spirally rolled type, metallic honeycomb bodies having a honeycomb structure have been proposed recently. Each of these replacement metallic honeycomb bodies has the structure that, in each minimum unit making up the metallic honeycomb body, namely, in each minimum unit composed in combination of a planar band and a corrugated band arranged in a contiguous relation, the minimum unit is maintained at opposite end portions thereof in contact with an inner wall of an associated metallic casing. S-shaped or X-wrapped, metallic honeycomb bodies are proposed, for example, in Japanese Patent Application Laid-Open (Kokai) Nos. SHO 62-273051, HEI 1-218637 and HEI 4-227855.
In a metallic carrier body of the spirally-rolled type described above, each minimum unit is maintained at only one end portion thereof in contact with the inner wall of an associated metallic casing. In a metallic carrier body of the last-mentioned conventional type, however, each of a desired plural number of minimum units is maintained at both opposite end portions thereof in contact with the inner wall of an associated metallic casing. It is therefore possible to absorb or reduce a thermal stress at each end portion, so that improved durability is obtained.
Incidentally, the terms employed above to indicate the various shapes of the above-described metallic honeycomb bodies may be regarded as common names as will be described next.
Paying attention to one of the minimum units of a metallic honeycomb body, that is, to one multilayer unit consisting of a planar band and a corrugated band, the metallic honeycomb body is called an "S-shaped" metallic honeycomb body where the multilayer unit is curved in the form of letter S when the metallic honeycomb body is viewed at a front face thereof. In a metallic honeycomb body fabricated by making a desired number of stacks, typically four stacks--each being formed of such minimum units superposed one over the other in a desired number of layers--abut at one end portions thereof and then rolling the individual stacks about the abutting end portions thereof, the abutment of the four stacks is in an X-shaped (or swastika) pattern. Metallic honeycomb bodies of this type are therefore called "X-wrapped" metallic honeycomb bodies.
S-shaped or X-rapped metallic honeycomb bodies, which are different from those of the above-described spirally-rolled type, are accompanied by the following drawbacks:
(i) In an S-shaped metallic honeycomb body, each minimum unit consisting of a planar band and a corrugated band unavoidably includes sites where the bands are folded over 180.degree.. Especially, the waveform of the corrugated band is therefore deformed because of the application of working stresses. It is therefore impossible to fabricate a metallic honeycomb body having uniform cells. PA1 (ii) In an X-wrapped metallic honeycomb body, the state of abutment of each stack at a central part is such that the end portions of the individual planar band and corrugated bands of the stack are arranged in abutment against a planar band forming an outermost layer of the adjacent stack. In other words, the end portions of the individual bands forming each stack are abutted and fixed by brazing or the like at edges thereof in a point-to-point fashion to an outermost planar band of the adjacent stack. The mutual abutment and fixed securement of the individual stacks are not sufficient and, when deforming force due to a thermal stress, vibrations or the like is applied to the abutted portions, the durability of the metallic honeycomb body is significantly deteriorated. PA1 (i) each of the stacks has a notched fitting portion so that the stacks are mutually fitted together at the notched fitting portions thereof; and PA1 (ii) the stacks have been rolled in the same direction about the notched fitting portions thereof as a central axis of the rolling formation.
As has been described above, metallic honeycomb bodies other than those of the spirally-rolled type, especially X-wrapped metallic honeycomb bodies therefore require special consideration to the durability of their central parts because of their structures.